Graphene is the name denoted to a single sheet (1 atom thick) of sp2-bonded carbon atoms. Effectively this is a single sheet of graphitic carbon. However, the term graphene has also come to be used to describe in a more general sense thin films (1-40 layers) of sp2-bonded carbon under the auspices of discussions of single layer interactions and electrical behavior. Due to the difficulty in mass producing graphene films, it remains one of the most expensive materials known, with costs of over $1000 for a micron-sized sample as of April 2008. There are currently six known and reported methods for graphene film creation: mechanical exfoliation from graphite sources, such as natural, Kisch and highly-ordered pyrolytic graphite (HOPG), epitaxial growth on a graphene template, metal-catalyzed growth on copper and nickel foils, sodium reduction of ethanol, hydrazine reduction, and the sublimation of silicon from the surface of silicon carbide. Currently, the most widely used method is the exfoliation process from a Kisch or natural graphite substrate; however, reproducibility in the number of layers, uniformity in the layer thickness and layer widths make this method the least promising for mass production of graphene-based electronics. Epitaxial growth on SiC and metal-catalyzed graphene growth are currently the most promising techniques for large-scale graphene film creation. Large area sheets (>1 cm2) have been created using chemical vapor deposition on both thin nickel and copper catalyst foils. Procedures for the transfer of these films in solution have been developed, however, the thickness of these films tends to be very large and the transfer process is destructive, requiring dissolving the metal catalyst for each sample created. Further, the carrier mobilities observed in these films tend to be reduced with respect to the values found in graphene films created by either epitaxial growth on SiC or via the mechanical exfoliation technique. Epitaxial growth graphene on SiC occurs via the sublimation of silicon from the surface of of either 6H-SiC or 4H-SiC substrates, epitaxial layers, or 3C-SiC epitaxial layers. This process involves heating the SiC substrate to temperatures in excess of 1100° C., typically between 1500-1600° C. As the silicon is sublimed from the surface, the residual carbon reconstructs, forming sp2 bonded layers stacked on top of one another. This process is capable of creating large-area graphene films, with the limitation derived simply by the size of the largest available SiC substrates (currently, substrates up to 4″ in diameter are available commercially). The graphene layers are typically grown on low-offcut substrates. A hydrogen anneal of the SiC surface is often performed to smooth the SiC surface prior to the graphene film formation. There are two faces to the SiC substrate, a silicon- and a carbon-terminated surface. In the case of the former, thinner films (1-5 layers typically) are created, albeit with lower carrier mobilities and carrier densities. The growth of epitaxial graphene on the carbon face of SiC typically results in a larger number of monolayers, with as many as 30 or more graphene layers being grown. However, the carrier mobility is typically two to eight times higher in the graphene layers formed on the carbon face than on the silicon face. Due to the large number of layers on the carbon face, it can be difficult to modulate the conductivity of the carriers participating in the transport in the graphene layers through the use of field-effect gate modulation. An approach to reduce the number of graphene layers on the carbon face is therefore desirable.
There are a host of device applications for which graphene is highly desirable. Due to its two-dimensional structure, the electrical properties of the material are highly sensitive to modifications to its surface. Therefore, it is an ideal candidate for single molecule gas sensors. Because of the very high carrier mobility and low noise, graphene is desired as the channel material within field effect transistors, thereby enabling operation at very high frequencies. The high optical transparency of graphene also make it an ideal candidate for use as a transparent conducting electrode, which is required for optoelectronic devices where optical emission from underlying active regions would need a highly conductive, yet transparent contact to enable the emitted light to emit from the surface of the device. Currently, indium tin oxide is the material of choice for emitters in the visible region of the electromagnetic spectrum, but it is brittle, and therefore the high strength and flexibility of graphene would enable a host of other possible devices as well as provide significant benefit in current applications where indium tin oxide is used (lower sheet resistance, precise control of light transmission through varying thin film thickness, etc. . . . ). Finally, graphene has enabled devices such as a frequency multipliers, ultracapacitors, microelectromechanical systems (MEMS), and biocompatible sensors.
While the above devices and applications hold very high promise, all of the current processes for creating thin graphene layers lack the ability to create large-area graphene films of a few layers on a substrate amenable to cost-effective device fabrication and operation. Therefore, for all of these creation methods, a mechanism enabling the transfer of large areas of graphene to a substrate of interest is required.
Currently the only alternatives for transfer involve exfoliation or solution-based removal of the growth (donor) substrate followed by a subsequent transfer of the film in solution. In the case of the former, this involves the use of a non-releasable adhesive tape that is placed in contact with the graphene surface and is peeled away. There is no control over the graphene thickness or uniformity removed and the transfer procedure involves repeatedly placing the tape and graphene onto the desired substrate and peeling away, leaving graphene films with non-uniform thicknesses, morphologies, sizes and shapes. This process does not appear at present to be amenable to large scale graphene transfer, which is required for cost-effective mass production of graphene-based electronics and materials. Further, this process is not amenable to a single-flip process and therefore only the exposed graphene surface may be accessed. The second, solution-based process involves dissolving the donor substrate on which the graphene is grown, leaving the graphene film floating in the etchant solution. This film may then be transferred to a handle substrate of interest by placing the substrate within the solution and then pulling it through the floating graphene film. There are many difficulties with this transfer procedure. First, it is a destructive technique, involving the etching of the underlying substrate, which is costly. For epitaxial graphene grown on SiC this process is not possible, as SiC is not known to be readily soluble in any solution in its bulk form. Further this implies that the desired substrate for transfer and the solution used for etching of the initial substrate are compatible. Second, this process involves transferring of a graphene film in solution by floating the film on the solution surface and then pulling the handle substrate up through the solution, catching the graphene film on the substrate as it is removed from the solution. This process would presumably induce wrinkling, folding or tearing of the graphene films during the transfer process.